Echo suppressor



y Feb. 4, 1930.,l

H. c.' SILENT ECHO sUPPREssoR Filed Nov. .14, 1928 lNvi-:NToR

I6 Sale/@6 k ATTORNEY Patented Feb. 4,l 1930 UNITED.- STATES PTENTOFFICE HAROLD c. SILENT, oELAn'cmuoNT, NEW Yonx, AssI'GNoE rro AMERICANTELEPHONE AND TELEGEAEH ooiarANY, A CORPORATION or NEW Yonx nono.sUPrnEssoEA Application mea November 14, 132s. serial Nu. 319,323.

This invention vrelates to two-wayytransmission 'circuits and moreparticularly to the suppression of echoes in four-wire circuits having one two-Wire path adapted for transmission in one direction and a secondtwowire path adapted for transmission in the opposite direction. Y v Theobject of the invention is the` suppression of echoes in either path ofthe four- Ewire circuit to a degree varying directly with. .the'amplitude of the original waves' which exist .in the opposite pathand-with which are originated'the' echoes that it is desired tosuppress. l Y

In accomplishin the above-stated object,

the applicant provides means in each two-wire path for introducing aloss therein and means associated with said meansand. with the op-Iposite path for' re ulating said means to pro-f duce a loss whic variesvdirectly with the amplitudeof thewaves in the opposite path.

ance with the invention. Fig. 2 shows diagrammaticall an alternativearrangement of a part of t e circuit. y

With reference first to Fig. 1, there 1s lshown a section of afour-wirefcircuit comprising a two-wire line L adapted lfor transmissionfrom east to west and a two-wire line L adapted lfor transmission fromwest to east.

, In each of these lines,'there isY a one-way amplifier. In each paththe applicant employs a hybrid coil involving impedances to be disclosedmore fullyl hereinafter, andl :1s-v

sociated with each line is a ltuned amplifier. Between each tunedamplifier and the.hybrid .coil of the opposite transmission path,certain elements are arranged as will be more fully described inconnection-with the folto west over t e two-wire line L, the suppressionin the line L of echoes from the,

west end of the four-wire circuit will be accomplishcd in the followingmanner. The noise or speech is transmitted in the manner j.usual in theecho suppressing arrangements 'old in .the'art through the tunedamplifier associated with line L and through the trans. former T-to thetwo-electrode vacuum tubel rectifier RT, which includes in its circuits'a battery B- connected as shown. There is now impressed across theresistance R a voltage which is roughly proportional to the amplitude ofthe waves in the transmission path The hybridl coil H in thetransmission path L has connected on one side an artificial network,schematically shown, and on the opposite side ,two three-electrodevacuum tubes V and V1, having a common'plate battery B2'. As is wellunderstood in the art, thev ymore nearly the impedance of the vacuum theeater is the loss introduced 'into the line by the coil I-I. Theapplicant adjusts the network -connected to the coil H tubes balancesthe impedance of the network,

so that it will balance the impedance of the y tubes'V and V1 when thegrids of these tubes are at the most negative point and the flow ofplate current in the tubes at the minimum. The voltage impressed acrossthe resistance R by the waves in the line L with the battery i connectedas indicated will cause the rids of the tubes V ,and V1 to become' morenegative,and there'will he, of course, a corresponding reduction -in theplate current of l the tubes. AThis reduction of the plate currentincreases the impedance ofthe tubes and tends to balance the oppositelyconnected imrpedance of the network. Accordingly, as the grids ofthetubes V and' V1 are made more negative, thefloss introduced into theline L"-is increased, and'it will-readily be understood that thisincrease is roughly propor tional to the amplitude of the waves in thetransmission path L which have caused the l'voltage across resistance Rand the increase of negative voltage on the grids. Conversely, adecrease of amplitude' in the line I'jwill reduce the negative voltageimpressed of; the grids of tubes V-and V1 and consequently, the loss inthe hybrid con n' will be decreased. Instead of the arrangementdescribed above, an alternative arrangement may be employed as indicatedin part in Fig. 2. In this case, the plate current of the tubes V and V1(or the corresponding tubes), instead of beingnormally large, isnormally Zero, and the ynetwork connected to the hybrid coil of line L'is adjusted to balance the impedance of the tubes when the plate currentis brought to the maximum. The elements T, R'I and R, corresponding toelements T, RT and It of Fig. l, are connected as shown in Fig. 2, andatincreased voltage impressed across re.- sistance R, as the result ofan increase of amplitude of the waves in line L, will cause the grids ofthe tubes to become less negative. The results will be a iow of platecurrent in the tubes and a decrease of the impedance balancing thenetwork, wherebyLA the transmission loss in line L will be increased.

The purpose of theA condenser Gis to produce a hang-over action, wellunderstood in the art, to sustain the suppression in the line L untilthe ech'ohas-been completely suppressed.

By adjustment of the battery B, any desired threshold may be introducedinto the action of the rectifier tube R'I"t.hat is, this tube may bemade to function only when the amplitude of the waves in line L reachesor exceeds a given value. g

It will be understood, ofcourse, that the tubes V and V.l may be madevto operate on any desired part of their characteristic by` theadjustment of the battery B2.

It will readily be understood that the variable loss in the hybrid coilH of the line L Edependent upon the amplitude of' the waves in the lineL is produced by means of the elements T', RT, B', R', C', B1', V, V1and B2', corresponding to the elements introduced between the tunedamplifier'asso'ciated with line L and the hybrid coil H in the line L.

While the invention has been disclosed in 'one speciiic form which isdeemed desirable,

it will readily be understood that 'the' true invention is capable ofembodiment in many 'echoes which consists in introducing a lloss in eachpath and causing the loss in either path to vary directly with theamplitude of the waves in the opposite path.

2. In a four-wire circuit having a first path adapted for transmissionin one direction and a second `path adapted for transmission in theopposite direction, means in each path for introducing a transmissionloss therein and means associated with said means and with the oppositepath for causing said transmisson loss to vary directly with theamplitude of thewaves in said opposite path.

3. In a four-wire circuit having a rst path adapted for transmission inone direction andr a second path adapted for transmission in theopposite direction, means in one transmission path for introducing atransmission loss therein and means associated with said means and withthe opposite path for adjusting said means to produce a transmissionloss network, and means associated with the opposite transmission pathfor varying the im?l pedance of said variable impedance means directlywith the variation of amplitude of the waves in said opposite path.

5. In a four-wire circuit having a first path adapted for transmissionin one direction and a. second pathadapted for transmission in theopposite direction, a hybrid coil in each transmission path, anartificial network connected therto, a variable impedance meansconnected to said hybrid coil to balance said network, and meansassociated with the op-l posite transmission path for varying theimpedance of said variable impedance means directly with the variationof amplitude of the waves in said opposite path, said network beingdesigned to balance said variable impedance means at the point ofmaximum impedance. i

In testimony whereof,'I have signed my name to this specification this7th day of November, 1928.

HAROLD o. SILENT.

